1. Field of the Invention
The invention relates to a touch position coordinate detecting system, using acoustic waves propagating on a touch panel.
2. Description of the Related Art
Touch position coordinate detecting systems are usually used, for example, in graphic display apparatuses, generally provided with a detecting system for determining the coordinates of a touch position along two predetermined axis X and Y, which are generally at right angles. A touch, causing an acoustic attenuation, is generally realized by the application on the panel of a finger tip of a user, or alternatively of a stylus (referenced ST on FIG. 1) adapted for absorbing acoustic wave energy and manipulated by the user, thus selecting a displayed menu or command. The detection of the position of the touch along the coordinates enables the system to determine the choice of the user and possibly the next graphic to be displayed. Such a touch position coordinate detecting system is, for example, disclosed in European Patent No.: EP 0190734.
Referring to the prior art system shown in FIG. 1, an input transducer E launches on the touch panel TP an initial burst of acoustic waves having the same acoustic frequency F. The resulting acoustic beam travels along the X-axis and reaches an upper array UA of parallel reflective patterns. Each pattern is preferably angled at 45° with respect to the X-axis, and it reflects a portion of the initial acoustic beam amplitude towards an output transducer R, preferably via a lower reflective array LA, symmetrical to the upper array relative to the X-axis as shown in FIG. 1. Therefore, each acoustic path resulting from the double reflection on the two respective patterns UP and LP of the upper and lower reflective arrays is U-shaped. More particularly, the length of the U legs increases with the X coordinate of the reflective patterns. The output transducer receives the reflective portions of the acoustic beam with a delay between the successive portions, due to the travel time of the acoustic waves between two successive patterns.
Referring to FIG. 2, a time signal OS is developed from the output signal of the output transducer R. The initial transmitted burst IP has a finite duration T. The time abscissa t0 corresponds to the reception date of the shorter U legs acoustic beam (reflected by the first patterns p1 and p′1 of FIG. 1) and tN corresponds to the reception date of the longer U legs acoustic beam (reflected by the last patterns pN and p′N of FIG. 1). When a touch (or a stylus ST) is applied on the panel surface TP, an attenuation of the acoustic waves occurs in the region AA of the touch, resulting from an absorption phenomenon. More particularly, the touch partially absorbs the beam Bi reflected by the pattern pi having the coordinate xi. Therefore, a localized damping LD appears on the time signal of FIG. 2 at the time abscissa ti (see the dashed line of FIG. 2). The coordinate xi of the touch is determined from the time abscissa ti, using for example a formula of the type xi=C/2(ti−t0)+x1, where x1 is the X-coordinate of the first pattern p1 and C is the velocity of the acoustic waves.
Of course, for determining also the Y-coordinate of a touch, a further pair of arrays can be provided along the Y-axis, for example at the right edge and at the left edge of the panel as shown in FIG. 1.
As indicated above, the acoustic waves have the same frequency F, imposing a spacing value nλ between two successive patterns of an array, corresponding to an integral multiple of a wavelength at the frequency F. Moreover, as the acoustic waves are naturally attenuated during their travel, increasing the reflectivity of the arrays far from the transducers makes it possible to flatten the amplitude of the output time signal, thereby enhancing the signal-to-noise ratio for the higher X-coordinates. Therefore, the number of reflective patterns increases from an end of the array, near to the transducers, to the other end of the array, far from the transducers (e.g. the spacing between the reflective patterns and the integer n decreases when moving away from the transducers).
Another prior art document, U.S. Pat. No. 4,645,870, discloses a touch position coordinate detecting system, wherein the determination of a touch position is not realized from a time localized damping in the time output signal, but rather from a frequency damping in a frequency related output signal. The initial burst signal is composed of a number of frequency components f1, f2, . . . , fn, and the spacing between the successive patterns of the array is a function of the corresponding wavelengths λ1, λ2, . . . , λn, so that the array acts as a dispersive line and the frequencies f1, . . . , fn are spread and separated in the frequency related output signal. More details on the phenomena involved in the reflection of this type of array can be found in prior art documents relating to acoustic wave pulse compression filters, such as the document “The IMCON Pulse Compression Filter and its Applications”, by Tom A. Martin, IEEE Transactions on Microwave Theory and Techniques, Vol-MTT-21, No. 4, April 1973, pp. 186-194.
More particularly, in U.S. Pat. No. 4,645,870, the successive spacing between the reflective patterns varies along the X-axis from λ1 to λn. The position of each pattern in the array has to be very precise for enabling an appropriate spreading of the frequencies. Moreover, the thickness of each reflective pattern has to be very precise as well. Generally, the patterns are made of grooves formed in the edges of the panel, by an etching process. Therefore, the technique of touch position detection based on frequency measurements is not really compatible with a manufacturing process in series with the touch panels.
More generally, the input transducer periodically launches a burst of acoustic waves, for example every 0.5 millisecond. If the user does not touch the panel, the bursts are launched uselessly. It is well known that such touch position detecting systems are very energy consuming. The problem becomes crucial when such touch position detecting systems are provided on self-service terminals in public places, for example, in airports or train stations.
Moreover, returning again to European Patent No.: EP 0190734, in order to ensure a reliable detection with respect to the signal-to-noise ratio, the duration value T of each initial burst of acoustic waves has to be greater than W/C, in particular for 15″ panels, where W is the width of the reflective arrays, measured perpendicularly to the X-axis, and C is the velocity of propagation of the acoustic waves. In the embodiment described in the specification of the aforementioned European patent, the lower limit value of the duration T is 4 microseconds for obtaining a satisfactory signal-to-noise ratio of the output time signal. If such a burst has to be launched every 0.5 millisecond, the system consumes a huge amount of energy.
It is an object of the present invention to provide a touch position coordinate detecting system that alleviates the drawbacks of the prior art systems. More particularly, it is an object of the present invention is to provide a touch position coordinate detecting system having an improved accuracy in touch position detection.
Another object of the present invention is to provide a touch position coordinate detecting system that consumes a relatively small amount of energy while still providing satisfactory accuracy.
It is yet another object of the present invention to provide a touch position coordinate detecting system that can be manufactured easily in series.
Because the touch position detection of the present invention uses at least two frequency components in the received output signal, with regard to the system disclosed in European Patent No.: EP 0190734, the minimal threshold value for the duration T of the launched bursts according to the present invention can advantageously be divided in two, and, therefore, the corresponding energy consumption can be reduced accordingly.
Moreover, the touch position coordinate detecting system of the present invention does not require as precise a manufacture as needed by the system disclosed in U.S. Pat. No. 4,645,870, since the first and second interspersed sub-arrays can operate respectively at first and second frequencies in an overall stable way.